1. Field of the Invention
This invention relates to a set of electrophotographic toners which are used for electrophotographic machines such as electrostatic copying machines and laser printers.
2. Description of the Prior Art
In electrostatic copying machines (hereinafter referred to as copying machines) which utilize a binary developer comprising an electrophotographic toner (hereinafter referred to as a toner) and a carrier, an initial supply toner is contained together with a carrier in a developing apparatus from the beginning of the copying process, while a supplementary toner is contained in a toner hopper. The toner contained in the developing apparatus is mixed with the carrier in this apparatus, resulting in a developer. The developer formed in this way is supplied to a photosensitive drum by magnetic rollers. The toner contained in the toner hopper is supplied to the developing apparatus when a decrease in toner concentration in the developer contained in the developing apparatus is observed by a sensor for detecting toner concentration. Thereafter, the toner supplied to the developing apparatus is mixed with a carrier to form a developer which is then supplied to the photosensitive drum in the same way as mentioned above. The toner contained in the developer is adsorbed by an electrostatic latent image which has been formed on the photosensitive drum, and then transferred to paper, resulting in a desired toner image thereon.
The toner remaining on the surface of the photosensitive drum after the transfer to paper is scraped off the surface of the photosensitive drum by a cleaning blade in a cleaning apparatus.
In conventional copying machines, the supplementary toner contained in the toner hopper is the same as that contained in the developing apparatus from the beginning of the copying process. Usually, the toner mentioned above is composed of toner powder and additives adhering to the surface of the toner powder. This toner powder is obtained by mixing the material used, kneading the mixture, grinding the mixture into powder, and sizing the resulting powder to have a desired mean particle size. As the additives to be used for the toner, there can be mentioned, for example, charge-controlling agents and cleaning agents.
When a toner is used which has a cleaning agent adhering to the surface thereof, it is possible to prevent the toner from making a film on the surface of the photosensitive drum or on the surface of the carrier. Moreover, in the cleaning step, the toner can be readily removed from the surface of the photosensitive drum, and it is also possible to prevent the inversion of the cleaning blade by which the toner is scraped off the surface of the photosensitive drum, so that the cleaning of the surface of the photosensitive drum can be achieved satisfactorily.
However, in the initial stage of the copying process, the amount of cleaning agent in the toner is decreased, so that the cleaning blade tends to be inverted in the cleaning step, thereby making it impossible to remove the toner remaining on the surface of the photosensitive drum. To eliminate such disadvantages, it may be preferred to use a toner with an increased weight ratio of cleaning agent to toner powder. If such toner is used, it will be possible to prevent the inversion of the cleaning blade throughout the copying process. However, a great amount of cleaning agent is required in preparing the toner, which makes production cost significantly high.
In general, when the developer is subjected to a process for making a large number of copies by use of, for example, electrostatic copying machines, deterioration of the carrier in the developer tends to decrease the fluidity of the developer with repetition of the copying operation, so that the bulk density of the developer is increased gradually. This condition is observed as a deficiency of the toner by the sensor for detecting the toner concentration, and the toner is supplied from the toner hopper to the developing apparatus. Therefore, when increasing the number of copies, the amount of toner to be supplied to the developing apparatus is increased, so that the toner concentration in the developer becomes higher than that set in the initial stage of the copying process. Accordingly, conventional types of toner have the disadvantages of requiring increased consumption and of increasing the fog of toner images.
Recently, Japanese Laid-Open Patent Publication No. 55-157755 describes an improved type of electrophotographic toner in which silica adheres as an external additive to the surface of toner powder wherein the ratio of silica to toner powder in the initial supply toner is set to be smaller than that of silica to toner powder in the supplementary toner. However, similarly to the case when the conventional types of electrophotographic toner mentioned above are used, such an improved type of electrophotographic toner would not be able to prevent sufficiently the inversion of the cleaning blade in the initial stage of the copying process. Moreover, when increasing the number of copies, the bulk density of the developer contained in the developing apparatus is increased gradually, so that the toner concentration in the developer becomes high, which gives rise to many disadvantages as mentioned above.